Ozone Depletion - Misconceptions About Ozone Depletion

Misconceptions About Ozone Depletion

CFCs are "too heavy" to reach the stratosphere

Since CFC molecules are heavier than air (nitrogen or oxygen), it is commonly believed that the CFC molecules cannot reach the stratosphere in significant amount. But atmospheric gases are not sorted by weight; the forces of wind can fully mix the gases in the atmosphere. The CFCs are evenly distributed throughout the turbosphere and reach the upper atmosphere.

Man-made chlorine is insignificant compared to natural sources

Another misconception is that "it is generally accepted that natural sources of tropospheric chlorine are four to five times larger than man-made one". While strictly true, tropospheric chlorine is irrelevant; it is stratospheric chlorine that affects ozone depletion. Chlorine from ocean spray is soluble and thus is washed by rainfall before it reaches the stratosphere. CFCs, in contrast, are insoluble and long-lived, allowing them to reach the stratosphere. In the lower atmosphere, there is much more chlorine from CFCs and related haloalkanes than there is in HCl from salt spray, and in the stratosphere halocarbons are dominant . Only methyl chloride which is one of these halocarbons has a mainly natural source, and it is responsible for about 20 percent of the chlorine in the stratosphere; the remaining 80% comes from man made sources.

Very violent volcanic eruptions can inject HCl into the stratosphere, but researchers have shown that the contribution is not significant compared to that from CFCs. A similar erroneous assertion is that soluble halogen compounds from the volcanic plume of Mount Erebus on Ross Island, Antarctica are a major contributor to the Antarctic ozone hole.

An ozone hole was first observed in 1956

G.M.B. Dobson (Exploring the Atmosphere, 2nd Edition, Oxford, 1968) mentioned that when springtime ozone levels over Halley Bay were first measured in 1956, he was surprised to find that they were ~320 DU, about 150 DU below spring levels, ~450 DU, in the Arctic. These, however, were at this time the known normal climatological values because no other Antarctic ozone data were available. What Dobson describes is essentially the baseline from which the ozone hole is measured: actual ozone hole values are in the 150–100 DU range.

The discrepancy between the Arctic and Antarctic noted by Dobson was primarily a matter of timing: during the Arctic spring ozone levels rose smoothly, peaking in April, whereas in the Antarctic they stayed approximately constant during early spring, rising abruptly in November when the polar vortex broke down.

The behavior seen in the Antarctic ozone hole is completely different. Instead of staying constant, early springtime ozone levels suddenly drop from their already low winter values, by as much as 50%, and normal values are not reached again until December.

The ozone hole should be above the sources of CFCs

Some people thought that the ozone hole should be above the sources of CFCs. However, CFCs are well mixed globally in the troposphere and the stratosphere. The reason for occurrence of the ozone hole above Antarctica is not because there are more CFCs concentrated but because the low temperatures help form polar stratospheric clouds. In fact, there are findings of significant and localized "ozone holes" above other parts of the earth.

The "ozone hole" is a hole in the ozone layer

There is a common misconception that the “ozone hole” is really a hole in the ozone layer. When the "ozone hole" occurs, the ozone in the lower stratosphere is destroyed. The upper stratosphere is less affected, so that the amount of ozone over the continent decreases by 50 percent or even more. The ozone does not disappear through the layer, nor is there a uniform 'thinning' of the ozone layer. It is a "hole" which is a depression, not in the sense of "a hole in the windshield."